1. Field of the Invention
This invention relates to depth controllers such as are used in marine seismic prospecting for controlling the depth of seismic marine streamer cables.
2. Description of the Prior Art
In marine seismic surveying by the use of seismic streamer cables, it is the present practice to mount a plurality of spaced-apart depth controllers throughout the length of the cable to maintain it within a predetermined depth range. A commonly-used depth controller is described in U.S. Pat. No. 3,375,800. Other type controllers are described, for example, in U.S. Pat. Nos. 3,531,761 and 2,531,762.
The known depth controllers, especially those that are now in wide use, are characterized by a pair of vanes rotatable about a fixed horizontal axis perpendicular to the streamer cable. The vanes assume a particular climbing or diving angle, dependent on the instantaneous depth within the water of the seismic cable section to which the controller is attached. The vanes' rotation about the horizontal axis is controlled, by a pressure-responsive device. When the cable's controller falls below a reference depth, its vanes rotate into a position to produce positive lift and the controller will lift the cable; on the other hand, when the cable climbs above the reference depth, the vanes rotate to produce negative lift and the controller will lower the cable. In this manner, the cable is automatically maintained within a prescribed depth range.
In any prior art controller with vanes rotating about a horizontal axis perpendicular to the controller's body, the chief drawback is that at any depth, especially at the cable's operating depth, the vanes remain completely outside of the controller's body. Fully exposed vanes generate turbulence and contribute appreciably to the undesired noise detected by the hydrophones within the streamer cable. Other known drawbacks are: the fully-exposed vanes have a tendency to snag debris, seaweed, etc. Such foreign matter may foul the vanes and interfere with their functioning, especially when the foreign matter becomes wedged between the vanes and the body. Often, the vanes become snagged on a reef or wreck, thereby completely and abruptly stopping the cable while under tow and causing the cable to become severed. Under such circumstances, a part or all of a very expensive streamer cable may be irretrievably lost. An additional, though relatively minor disadvantage of such prior art controllers is that their storage aboard ship requires excessive space, since their vanes cannot be retracted into the bodies of the controllers.